Apple's iPad 3G went on sale late Friday afternoon and while Apple hasn't announced any sales numbers, Piper Jaffray & Co. analyst Gene Munster offers a conservative estimate of 300,000 units sold between Friday and Sunday. This is the same analyst who initially estimated 600,00-700,000 units units sold on the WiFi iPad launch day, after which Apple announced that only 300,00 were sold. According to Fortune, this is why Munster is now reporting his most conservative numbers. His forecast is 1.3 million units sold by the end of June.

Munster's team called 50 Apple stores on Sunday and found that 49 were sold out of the 3G model, and most were out of the WiFi model as well. VentureBeat reports that Best Buy was also virtually sold out by Sunday. As of this morning, Apple's online store is reporting a ship date of 5-7 days for all iPad models.
Of course shortages always beg the question of whether they're caused by too low inventory or unexpectedly high demand. Or even whether the shortage is deliberate to drive demand. People always want what they can't get, after all.
[Update: CNET just put up a post saying that Apple reports its one millionth iPad was sold on Friday. That 1.3 million figure may have already been hit, and certainly will be well before the end of June.]
In other iPad news, the Spirit jailbreak is now available for downloading. It'll jailbreak the iPad as well as the iPhone and iPod Touch, though we're warned "On iPad, all this is still sort of beta." In particular, people seem to be losing their Camera Roll during the jailbreak process, so if you decide to take the plunge, be sure to have a back up of your photos and turn off auto-sync in iTunes; you don't want to sync your jailbreak over your backed up data.